Pollution of underground water by lead has been taken as an environmental pollution these days, and products including lead are severely restricted, so that tin-lead solder is replaced by lead-free solder. Thus, plating layers coated with the tin-lead solder should be replaced by the lead-free solder.
Tin-silver-system alloy will be employed instead of the tin-lead solder alloy, so Matsushita Electric Company disclosed tin-silver solder paste (see Nikkei Sangyo Press, Feb. 1, 1996). A method of forming the tin-silver solder layer is required now. But difference of electrodeposition potential between silver and tin is 900 mV or more as standard oxidation-reduction potential, the cyanide, e.g., potassium cyanide, is included in plating solution so as to codeposite tin and silver when forming the tin-silver alloy layer. With the cyanide, there are many problems of polluting waste water, safe work, etc., so the tin-silver plating solution including no cyanides is required.
On the other hand, alloys which are made by adding bismuth, copper, etc. to the tin-silver alloys have better soldering characteristics, so the tin-silver-system alloy plating solution has been required.
An object of the present invention is to provide the tin-silver-system alloy plating solution, which is capable of being employed instead of the tin-lead alloy plating solution, including no cyanides.